Just to clarify - it is very hard to extend 'Basic' reserve into days off outside of extenuating circumstances such as cancelation or -117 issues at an outstation. That is of course assuming that the Days off are HDOs. On 'Basic' reserve, all your days off are HDOs with one FDO per month. They can fly you into an FDO without prejudice (it does get restored).

The above synopsis of line holder re-asignemnt is spot on as far as 'what they can do' - also like it was said, it is very Base / Equipment / Seat dependent too. That being said, the MEC website has an exhaustive discussion on how our line holder reassignment rules differ (are better in a lot of ways) than AA or DL.